1. Field of the Invention
The present invention is directed generally to drive circuits and more specifically to drive circuits for thin film electroluminescent (TFEL) edge emitter devices.
2. Description of the Prior Art
As is known, TFEL elements emit light when a changing electric field is present across the element. TFEL elements are thin film structures comprised, for example, of a phosphor layer situated between dielectric composite layers and electrodes located on the outside of the dielectric layers. Examples of the structure of TFEL elements are found in U.S. Pat. Nos. 4,535,341, 4,734,723, and U.S. Pat. application Ser. No. 273,296, filed 18 Nov. 1988 now U.S. Pat. No. 5,004,956 and assigned to the same assignee as the present invention.
TFEL edge emitter devices are typically configured in arrays utilizing multiplexed common drive circuitry to control many TFEL devices from a single source. A common driver circuit generates the peak to peak voltage necessary to illuminate the TFEL edge emitter device, and a demultiplexing channel driver directs that signal to the individual devices as desired. Work is being performed to improve edge emitter structures so that fewer physical connections between the individual pixels carried on each device and the driver circuitry are required. An example of such work is U.S. Pat. application Ser. No. 343,697, filed Apr. 24, 1989, now U.S. Pat. No. 4,899,184, and assigned to the same assignee as the present invention.
TFEL devices require large operating voltages on the order of five hundred volts peak to peak. Not only is a high voltage required for operating the device, but the voltage must be precisely regulated to obtain consistent, reproducible, light-emission from the TFEL device. Presently, the common driver circuits use current steering transistors to charge the capacitive TFEL load. The voltage stored in the TFEL load is then discharged by dissipating the power through resistive circuitry. See, for example, FIG. 6 of the aforementioned U.S. Pat. application Ser. No. 343,697 and the accompanying description thereof.
As the applications for TFEL devices increase, and with the potential for portable applications, the need exists for an efficient common drive circuit. In addition to achieving efficient use of supply power, the drive circuit must also be capable of precisely regulating the applied voltage whenever the supply voltage is not precisely controlled.